Recently, the third generation partnership project approved a study (3GPP TD RP-141664: “Study on Licensed-Assisted Access using Long Term Evolution (LTE)” to evaluate the LTE enhancements for Licensed Assisted Access (LAA) to an unlicensed spectrum. The objective of the 3GPP study is to define evaluation methodologies and deployment scenarios for the LTE deployments focusing on a LTE carrier aggregation and identifying enhancements to the LTE; meeting the requirements and targets for the unlicensed spectrum deployments. Further, the study proposes that a Base Station (BS) can use the unlicensed spectrum through the carrier aggregation. In the study, 3GPP has come up with dynamic channel access strategies to access the unlicensed spectrum.
In existing systems, IEEE 802.11 wireless LAN (WLAN) stations operate on an unlicensed band and Distributed Coordination Function (DCF) is the basic mode of operation for accessing the channel. The DCF provides a fair access to all 802.11 stations by doing a channel sensing before transmission with an exponential back off. The mechanism is known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Further, the channel sensing ensures that no other 802.11 station is transmitting in the channel (i.e., to avoid direct collision) and the exponential back off ensures that the opportunity of getting the channel is shared evenly among all stations.
For example, consider the 802.11 WLAN where “M” stations have to communicate by using the DCF procedure. For simplicity, 4 stations are shown in the FIG. 1a. As shown in the FIG. 1a, packet arrival to a Media Access Control (MAC) can happen at any time. The station (i.e., station 1 as shown in the FIG. 1a) wants to transmit performs a Clear Channel Assessment (CCA) for a DCF inter frame space (DIFS) time. If the channel is idle then, the station will choose a random number (i.e., back off counter) “N” as the back off time slots from 0 to Contention Window (CW). The CW is initially taken as equal to a minimum contention window (CW_MIN). The back off counter “N” is decremented by one every time the channel is idle for slot duration. In case when the stations determine that the channel is busy before the back off counter “N” becomes zero then, the stations will not alter the back off counter “N”. After the next DIFS time, the stations will decrement the counter from the last value, instead of choosing a new one. When the back off counter Ni reaches zero then the stationi (i.e., STAi) transmits its frames by accessing the channel. In the FIG. 1a, the station 1 transmits after the back off counter becomes zero.
In other existing systems, Listen before Talk (LBT) is a channel access process followed in the unlicensed bands. The channel is observed for a fixed amount of time. At the end of the observation time, decision to transmit or occupy the channel is determined based on the channel being idle or busy. Thus, ensuring that the station that wants to transmit does not interfere (i.e., avoid a collision) with another station that is already occupied the channel.
In other existing systems, Load Based Equipment (LBE) implements an LBT based spectrum sharing mechanism based on the CCA mode using “energy detect”. The LBE can utilize any of the following two mechanisms i.e., fixed back off or exponential back off, before occupying the unlicensed spectrum.
In the exponential back off, the CCA is performed before the transmission. At least for a time period of 20 μs, the channel should be idle. In case the energy level in the channel is above the pre-determined threshold within the CCA time then the channel is considered as busy. The station which determines the channel to be busy will not do any transmission, instead it will perform an Extended CCA (ECCA) mechanism in which the channel is observed for “Qi” observation slots (i.e., an observation slot can be an unoccupied idle slot of 18 μs (ECCA slot time) or a busy slot). The busy slot is the total time in which the channel was found occupied in between two unoccupied ECCA slots and it should be considered as the single observation slot.
In the fixed back off, the ECCA should be carried out in the channel for the observation period which is the random number Ni multiplied by the CCA observation time of 20 μs, where Ni is the number of unoccupied idle slots that are required to be observed before initiating the transmission. The value of Ni is randomly selected in the range 1 to “Qi” every time the ECCA is required and the value is stored in a counter. The value of “Qi” is selected by the manufacturer in the range 4 to 32. The counter is decremented every time the CCA slot is considered to be unoccupied. Whenever the counter reaches zero, the station may occupy the channel.
The above information is presented as background information only to help the reader to understand the present invention. Applicants have made no determination and make no assertion as to whether any of the above might be applicable as Prior Art with regard to the present application.